Phage therapy against Pseudomonas aeruginosa infections has received renewed attention owing to the increasing prevalence of antibiotic resistance in this bacterium. Here, we isolated and characterized two new potentially lytic bacteriophages (MPK1 and MPK6), which produced large and clear plaques on P. aeruginosa strain PAO1. Based on their morphology, MPK1 belongs to the Myoviridae, while MPK6 belongs to the Podoviridae. The group B polysaccharide of lipopolysaccharide was required for infection, suggesting that their host spectra are associated with the serotypes of P. aeruginosa strains. Intramuscular and intraperitoneal administration of MPK1 and, to a lesser extent, MPK6 significantly protected mice from mortality caused by PAO1-induced peritonitis-sepsis (P < 0.01). Mice treated with either phage also had lower bacterial burdens in their livers, lungs, and spleens. The antibacterial efficacy of MPK1 and MPK6 was also evaluated based on Drosophila melanogaster systemic infection caused by P. aeruginosa, for which phages were administered by feeding. Both phages significantly delayed the PAO1-induced killing of D. melanogaster (P < 0.001), although MPK1 persisted longer than MPK6 in uninfected D. melanogaster tissue samples. These results suggest that a mini-scale experiment using D. melanogaster infection is valid for evaluating the antibacterial efficacy of phage therapy against P. aeruginosa infections.